The present invention relates to an ink jet recording sheet, and particularly to an ink jet recording sheet which exhibits a small amount of ink bleeding, and results in high color density.
In recent years, ink jet recording has increasingly resulted in enhancement of image quality which is approaching that of conventional photography. In order to achieve such conventional photographic quality (also called a photo-grade quality), improvement has been made in the area of recording sheets. A void type recording sheet, which comprises a highly smoothened support having thereon a minute-void layer comprised of fine particles and hydrophilic polymers, exhibits high gloss, results in bright color formation, and exhibits excellent ink absorbability as well as ink drying properties. Accordingly, said recording sheet has been regarded as one of those which are most similar to conventional photographic quality.
In accordance with such requirements for high quality image, the demanded properties for ink jet recording sheets are increasing. Specifically, when a non-water absorptive support is employed, it is possible to maintain the highly smoothened surface, and a result high quality prints can be obtained.
Ink jet ink usually comprises water-soluble dyes.
However, ink, comprised of water-soluble dyes, results in disadvantages such as bleeding and poor water resistance due to the high hydrophilicity of said water-soluble dyes. Namely, when after printing, prints are stored at high humidity for a long period of time, or water droplets are allowed to adhere onto the print surface, said dyes tends to result in bleeding.
Ink jet recording sheets having a porous ink absorptive layer can yield a uniform image due to high ink absorptivity.
On the other hand, there occurs problem that the color density of images decreases due to the light scattering on the surface of fine particles which forms a porous layer.
In order to overcome such drawbacks, it is a general practice to incorporate dye fixing materials such as cationic materials into a porous layer.
A method preferably employed is one in which, for example, an anionic dye is allowed to bond to a cationic polymer so as to be immobilized. Cited as such cationic polymers are polymers of quaternary ammonium salts.
Japanese Patent Publication Open to Public Inspection No. 56-99693 discloses a method in which a cationic surface active agent is bonded to a water-soluble dye to insolubilize.
Further, Japanese Patent Publication Open to Public Inspection Nos. 55-53591, 55-150396, 56-867789, 58-89391, and 58-94491 describe ink jet recording sheets comprised of water-soluble polyvalent metal salts which bond to water-soluble dyes to form minimally water-soluble salts.
Japanese Patent Publication Open to Public Inspection No. 60-257286 describes an ink jet recording sheet comprised of basic polyhydroxyl aluminum compounds.
Further, Japanese Patent Publication Open to Public Inspection Nos. 60-67190, 61-10484, and 61-57379 describe ink jet recording sheets comprised of cationic polymers as well as water-soluble polyvalent metal salts.
However, above-mentioned methods are insufficient to improve water resistance and bleeding during storage. There are no description how to prevent the decrease the density of images printed on a porous ink absorptive layer of ink jet recording sheet.
Further, Japanese Patent Publication Open to Public Inspection No. 4-7189 discloses a method employing a porous pigment and an acid zirconium chloride compound. Said patent specification describes that by adding said acid zirconium chloride salt, desired adhesion strength is obtained employing a relatively small amount of binder, and it is possible to achieve improvement of image quality. But, here again, there is no description about the bleeding during storage or image density decrease.
Japanese Patent Publication Open to Public Inspection No. 6-32046 discloses a method in which a zirconium compound is combined with silica and modified polyvinyl alcohol. There remains problems of productivity and cost because this method uses specific polyvinyl alcohol. And there is no disclosure of ink bleeding during storage in this publication.
Further, Japanese Patent Publication Open to Public Inspection No. 10-258567 discloses a method employing a hydrophilic polymer and a water-soluble compound comprising a 4A group element, in combination, while Japanese Patent Publication Open to Public Inspection No. 10-309862 discloses a method employing a hydrophilic polymer together with a polyhydric carboxylic acid, and a zirconyl compound, also in combination. These references do not describe a porous ink absorptive layer. The ink absorptive layer made of non-porous hydrophilic polymer does not absorb sufficient amount of ink, and improvement of ink bleeding during storage is not enough. They does not describe about the effect on density of the printed image.
Further, European Patent No. 754,560 discloses that a water-soluble binder, a pigment, a zirconium compound, and a cationic polymer are employed in combination.
However, the above-mentioned prior arts do not disclose ways to minimize bleeding after keeping for a long time, or to yield high color density of the printed image.
An object of the present invention is to provide an ink jet recording sheet in which, after printing, the printed image results in high density and minimized bleeding during storage and improvement in water resistance.
Said objects as above were achieved employing the constitution described below.
1. An ink jet recording sheet comprising a support having thereon a porous ink absorptive layer comprising, inorganic particles, polyvinyl alcohol and a zirconium compound or an aluminum compound, wherein said ink absorptive layer exhibits a concentration gradient of said zirconium compound or said aluminum compound, wherein said concentration increases from the surface of said support towards the surface of said ink absorptive layer.
2. The ink jet recording sheet of item 1, wherein said zirconium compound or said aluminum compound satisfies the following requirement:
0xe2x89xa6C0.2/C0.8xe2x89xa60.8, 
wherein C0.8 and C0.2 are a concentration of said zirconium compound or said aluminum compound at a distance of {fraction (8/10)} and {fraction (2/10)} of a thickness of the ink absorptive layer from the surface of the support, respectively.
3. The ink jet recording sheet of item 1, wherein the ink absorptive layer comprises at least two layers, wherein an amount of said zirconium compound or said aluminum compound expressed in g/m2 in each of the two layers satisfies the following requirement:
0 less than CI/CIIxe2x89xa60.8, 
wherein CI is the amount of said zirconium compound or said aluminum compound in a lower layer and CII is the amount of said zirconium compound or said aluminum compound in an upper layer.
4. The ink jet recording sheet of claim 1, wherein an average diameter of primary particles of said inorganic particles is from 20 to 100 nm.
5. The ink jet recording sheet of claim 1, wherein a weight ratio of said inorganic particles to said polyvinyl alcohol is from 2:1 to 20:1.
6. The ink jet recording sheet of claim 1, wherein said support is a non-water absorptive support.
7. The ink jet recording sheet of claim 1, wherein said ink absorptive layer further comprises a cationic polymer.
8. The ink jet recording sheet of claim 1, wherein said polyvinyl alcohol is hardened with a hardener.
9. A method of preparing the ink jet recording sheet of item 1, comprising the steps of:
a) applying a coating composition comprising the inorganic particles and polyvinyl alcohol to the support so as to form a first coating layer; and
b) applying another coating composition comprising the zirconium compound or the aluminum compound on the first coating layer so as to form the ink absorptive layer.